Regarding electrophotographic image forming apparatuses, a charging method is known in which a photosensitive member as a charging object is charged by a corona charger as a charging device. In particular, a corona charger called scorotron is known. The scorotron includes a grid electrode for stabilizing the potential for charging the photosensitive member. In such a configuration, it is known that the grid electrode of the corona charger is provided in proximity to the photosensitive member so that uniform charging of the photosensitive member is performed efficiently.
Furthermore, in a typical internal configuration of the corona charger, a discharge electrode is enclosed by a housing that includes shields provided on two respective sides of the discharge electrode, and the housing has an open portion on a side thereof facing the photosensitive member as the charging object. It is known that a surface of the photosensitive member that faces a space between the shields of the corona charger is charged by corona discharge through the open portion of the housing.
On the other hand, it is known that discharge products such as ozone O3 and nitrogen oxides NOx are generated when corona discharge occurs in the air. If any discharge products adhere to the photosensitive member and, moreover, absorb moisture, the surface resistance in such a portion having the discharge products is lowered. This may trigger an image defect so-called “image deletion” in which an electrostatic latent image that is faithful to image information is not formed.
Accordingly, a configuration is disclosed by PTL 1 in which a photosensitive member and an open portion that is provided on a photosensitive-member side of a housing of a corona charger are shielded from each other by a shutter provided between the two and being movable openably and closably in the longitudinal direction of the charger. Specifically, the leading end of a sheet-type shutter in a closing direction is supported by a carriage, and the carriage is moved in the longitudinal direction of the corona charger, whereby the open portion and the photosensitive member are shielded from each other when image formation is not performed. Thus, adhesion of discharge products to the photosensitive member is suppressed. Furthermore, according to PTL 1, the shape of the shutter is regulated in the circumferential direction of the photosensitive member such that a central portion bulges toward the housing with respect to two ends.